[Laser] laser optics questions

Sun Mar 21 13:01:37 EDT 2010

Chris
I notice no problem with your mail format.
The paragraphs are without interligne, well separated and clear.
About laser ATV I agree it is possible to use LEDs at short range without problems but at long range, to cancel the scintillation you have to use a higher frequency to built an FM subcarrier.
http://www.mydarc.de/dj1wf/Laser/Harz/Harz1.html
73
Yves F1AVY
http://f1avyopto.wifeo.com 

--- En date de : Dim 21.3.10, Chris L <vocalion1928 at hotmail.com> a écrit :

> De: Chris L <vocalion1928 at hotmail.com>
> Objet: Re: [Laser] laser optics questions
> À: laser at mailman.qth.net
> Date: Dimanche 21 mars 2010, 17h15
> 
> To take your points in turn:
> 
> (1) Tim, you are confusing the concepts of spatial
> coherence and monochromaticity. Spatial coherence refers to
> the radiation from a laser source being in the form of plane
> waves, propagated in phase - "in step", if you like. Only a
> laser source, or a point source at very great distance from
> the observer can produce spatially coherent radiation.
> 
> Monochromaticity, on the other hand, refers to the
> BANDWIDTH of the waves propagated by the source. A highly
> monochromatic source can be either noncoherent or coherent -
> or coherent to an intermediate degree. The diffusing filter
> shown in Korotkova's papers removes the spatial coherence
> from the laser radiation, but leaves the source's
> monochromaticity (bandwidth) substantially unaffected. A
> beam can be highly monochromatic (like the output of a diode
> laser), but it need NOT be coherent. In fact, it is
> desirable that this spatial coherence be avoided for
> atmospheric transmission, as highly spatially coherent
> beams, in the presence of atmospheric turbulence, can
> encounter constructive and destructive interference
> producing extremely deep and rapid scintillation. Refer
> Korotkova's papers on the controlled removal of spatial
> coherence of an atmospheric laser comms beam to reduce the
> bit-error-rate of digital comms:
> 
> http://pegasus.cc.ucf.edu/~okorotko/SPIE4821.pdf
> http://pegasus.cc.ucf.edu/~okorotko/SPIE4976.pdf
> http://pegasus.cc.ucf.edu/%7Eokorotko/OptEng43.pdf
> 
> It may be difficult to grasp the fact that coherence is
> undesirable in atmospheric optical communication, but if you
> read the three papers above and refer to our success in
> applying those basic principles of atmospheric physics, I
> think you'll see the logic of what we're doing. Please do
> read these references. You need not undersrtand the math
> fully to appreciate the principles being expounded. We
> described the practical applications of these principles in
> our field tests in this paper for the SPIE in 2008:
> 
> http://www.modulatedlight.org/Dollars_vesus_Decibels_colour.pdf
> 
> (2) The scintillation encountered in beams received from
> LED-derived non-coherent beams collimated, at the tx and rx
> ends, by large-aperture lenses (and I'm talking 20 cm
> Fresnels here, or larger) is derived from sources other than
> decohering noise. The scintillation occurs owing to
> atmospheric effects including (a) beam wandering or
> steering, (b) beam focussing and defocussing, (c) changing
> angle of beam arrival or image jitter. For an excellent
> reference on atmospheric scintillation effects, I refer you
> to:
> 
> SPIE FIELD GUIDE SERIES, Volume FG02: "Atmospheric Optics"
> by Larry C Andrews; SPIE Press, Bellingham, Washington, USA,
> 2004. (refer website: 
> www.spie.org/press/fieldguides  )
> 
> (3) Tim, the usage of Fresnels for laser beam collimation
> is unsuitable for several reasons, and there are very good
> reasons for their not working properly with coherent
> sources, as Clint has already described on the basis of
> personal demonstration. (A) To adequately and predictably
> collimate a COHERENT beam without variations of constructive
> and destructive interference across the collimator area, the
> collimating optic must have a surface accuracy considerably
> finer than 1/4 wavelength. In other words, for laser
> collimation it must be a DIFFRACTION LIMITED optic.
> Practical molded Fresnels produce an image spot, or "blur
> circle" of between 0.25 and 0.5 mm. This is much too large
> for the diffraction-limited effective point source that a
> laser produces. So there is, in fact, very good reason for
> coherent (laser) sources producing more scattering, more
> beam inhomogenity and radomly unpredictable phase
> cancellation with Fresnels than there is from any poractical
> LED source.
> 
> (4) To Yves: Clint Turner (KA7OEI) demonstrated the
> transmission and reception of NTSC video signals via a
> Luxeon LED source in his workshop more than three years ago.
> There is NO problem in modulating high power Luxeon LED's to
> 5 or 6 MHz, though the higher powered PhlatLight LEDs have a
> somewhat higher junction capacitance. The problems begin to
> occur when one aims for a broader bandwidth than about 10
> MHz with present high power LED hardware.
> 
> Incidentally I'm trying one more form of layout for this
> posting, hoping that in doing so, I'm not going to run into
> the "line break" problem affecting my earlier postings. If
> this doesn't work, I've no idea what to do about the
> problem!
> 
> Chris Long VK3AML.
> 
> =================================
> 
> > Date: Sun, 21 Mar 2010 06:27:32 -0700
> > From: toasty256 at yahoo.com
> > To: laser at mailman.qth.net
> > Subject: Re: [Laser] laser optics questions
> > 
> > Hi all,
> > I just want to be clear, i'm not trying to argue one
> method is 
> > better than some other here. From what i've read about
> all this 
> > so far, it looks like most, if not all, of the
> de-coherence noise 
> > is generated in the first kilometer or two as the
> spatial 
> > coherence of the laser is being destroyed by passage
> through 
> > the atmosphere. The de-coherence process has, in
> effect, 
> > modulated the beam in amplitude and so the beam
> carries this 
> > noise with it to the detector. 
> > 
> > I suspect an LED still suffers from some scintillation
> because of 
> > its narrow bandwidth. Both the laser and LED have a
> finite 
> > bandwidth and so both have a degree of "coherence"
> which shows up 
> > as the noise and scintillation on both signals - the
> LED having 
> > less because its bandwidth is greater and has less
> temporal 
> > coherence. (Not because it has None) In the real
> world, no 
> > light source can be made "non-coherent" or infinite
> bandwidth.
> > 
> > I pick a random point in the discussion and run with
> it now... 
> > I don't think a fresnel lens would scatter laser light
> any more 
> > than it does with LED light. It may be that it is just
> more notifiable with a laser because it is a sharply defined
> 
> > reflection or scatter (caustic?). The LED light gets
> scattered 
> > just as much but its reflections are fuzzy and diffuse
> due to 
> > the wider bandwidth.
> > 
> > -toast
> > 
> >  
> > 
> > 
> > 
> > 
> > 
> >       
> >
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